Organic element for electroluminescent devices

ABSTRACT

Disclosed is an electroluminescent device comprising a light-emitting layer including a light emitting material that contains an organometallic complex comprising (1) a metal selected from the group consisting of Ir, Rh, Os, Pt, and Pd and (2) a diazole group ligand wherein the ligand has a fused aromatic ring group including a nitrogen of the diazole as a bridgehead nitrogen. The device provides useful light emission.

FIELD OF THE INVENTION

This invention relates to an organic light emitting diode (OLED)electroluminescent (EL) device comprising a light-emitting layercontaining an organometallic complex that can provide desirableelectroluminescent properties.

BACKGROUND OF THE INVENTION

While organic electroluminescent (EL) devices have been known for overtwo decades, their performance limitations have represented a barrier tomany desirable applications. In simplest form, an organic EL device iscomprised of an anode for hole injection, a cathode for electroninjection, and an organic medium sandwiched between these electrodes tosupport charge recombination that yields emission of light. Thesedevices are also commonly referred to as organic light-emitting diodes,or OLEDs. Representative of earlier organic EL devices are Gurnee et al.U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No.3,173,050, issued Mar. 9, 1965; Dresner, “Double InjectionElectroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334,1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Theorganic layers in these devices, usually composed of a polycyclicaromatic hydrocarbon, were very thick (much greater than 1 μm).Consequently, operating voltages were very high, often >100V.

More recent organic EL devices include an organic EL element consistingof extremely thin layers (e.g. <1.0 μm ) between the anode and thecathode. Herein, the term “organic EL element” encompasses the layersbetween the anode and cathode. Reducing the thickness lowered theresistance of the organic layer and has enabled devices that operatemuch lower voltage. In a basic two-layer EL device structure, describedfirst in U.S. Pat. No. 4,356,429, one organic layer of the EL elementadjacent to the anode is specifically chosen to transport holes,therefore, it is referred to as the hole-transporting layer, and theother organic layer is specifically chosen to transport electrons,referred to as the electron-transporting layer. Recombination of theinjected holes and electrons within the organic EL element results inefficient electroluminescence.

There have also been proposed three-layer organic EL devices thatcontain an organic light-emitting layer (LEL) between thehole-transporting layer and electron-transporting layer, such as thatdisclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616,1989]. The light-emitting layer commonly consists of a host materialdoped with a guest material, otherwise known as a dopant. Still further,there has been proposed in U.S. Pat. No. 4,769,292 a four-layer ELelement comprising a hole-injecting layer (HIL), a hole-transportinglayer (HTL), a light-emitting layer (LEL) and an electrontransport/injection layer (ETL). These structures have resulted inimproved device efficiency.

Many emitting materials that have been described as useful in an OLEDdevice emit light from their excited singlet state by fluorescence. Theexcited singlet state is created when excitons formed in an OLED devicetransfer their energy to the excited state of the dopant. However, it isgenerally believed that only 25% of the excitons created in an EL deviceare singlet excitons. The remaining excitons are triplet, which cannotreadily transfer their energy to the singlet excited state of a dopant.This results in a large loss in efficiency since 75% of the excitons arenot used in the light emission process.

Triplet excitons can transfer their energy to a dopant if it has atriplet excited state that is low enough in energy. If the triplet stateof the dopant is emissive it can produce light by phosphorescence,wherein phosphorescence is a luminescence involving a change of spinstate between the excited state and the ground state. In many casessinglet excitons can also transfer their energy to lowest singletexcited state of the same dopant. The singlet excited state can oftenrelax, by an intersystem crossing process, to the emissive tripletexcited state. Thus, it is possible, by the proper choice of host anddopant, to collect energy from both the singlet and triplet excitonscreated in an OLED device and to produce a very efficient phosphorescentemission.

One class of useful phosphorescent materials are transition metalcomplexes having a triplet excited state. Forexample,fac-tris(2-phenylpyridinato-N,C^(2′))iridium(III) (Ir(ppy)₃)strongly emits green light from a triplet excited state owing to thelarge spin-orbit coupling of the heavy atom and to the lowest excitedstate which is a charge transfer state having a Laporte allowed (orbitalsymmetry) transition to the ground state (K. A. King, P. J. Spellane,and R. J. Watts, J. Amer. Chem. Soc., 107, 1431 (1985), M. G. Colombo,T. C. Brunold, T. Reidener, H. U. Gudel, M. Fortsch, and H.-B. Burgi,Inorg. Chem., 33, 545 (1994)). Small-molecule, vacuum-deposited OLEDshaving high efficiency have also been demonstrated with Ir(ppy)₃ as thephosphorescent material and 4,4′-N,N′-dicarbazole-biphenyl (CBP) as thehost (M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R.Forrest, Appl. Phys. Lett., 75, 4 (1999), T. Tsutsui, M.-J. Yang, M.Yahiro, K. Nakamura, T. Watanabe, T. Tsuji, Y. Fukuda, T. Wakimoto, S.Miyaguchi, Jpn. J. Appl. Phys., 38, L1 502 (1999)).

Another class of phosphorescent materials include compounds havinginteractions between atoms having d¹⁰ electron configuration, such asAu₂(dppm)Cl₂ (dppm=bis(diphenylphosphino)methane) (Y. Ma et al, Appl.Phys. Lett., 74, 1361 (1998)). Still other examples of usefulphosphorescent materials include coordination complexes of the trivalentlanthanides such as Tb³⁺ and Eu³⁺(J. Kido et al, Appl. Phys. Lett., 65,2124 (1994)). While these latter phosphorescent compounds do notnecessarily have triplets as the lowest excited states, their opticaltransitions do involve a change in spin state of 1 and thereby canharvest the triplet excitons in OLED devices.

Notwithstanding these developments, there remains a need for neworganometallic materials that will function as phosphorescent emitters.M. E. Thompson and co-workers have reported on the synthesis andcharacterization of a number of iridium complexes (J. Amer. Chem. Soc.,125, 7377 (2003)). Many of the phenylpyridyl-based complexes wereintensely luminescent at room temperature. In contrast, thephenylpyrazolyl-based compounds, such as,tris(1-phenylpyrazolato-N,C^(2′)) iridium(R) either did notphosphoresce, or were very weak emitters, at room temperature.

Phenylpyrazolyl-based organometallic compounds are also described in JP2003109758 A2, JP 2003142265 A2, U.S. 2002134984 A1, and U.S.2002117662A1. U.S. 20030175553 A1 describes the use offac-tris(1-phenylpyrazolato-N,C 2)iridium(III) (Irppz) in anelectron/exciton blocking layer. In this case, the material is not anemissive dopant but is used to prevent the movement of electrons orexcitons from the luminescent layer into the hole-transporting layer.

It is a problem to be solved to provide new phosphorescent emittingmaterials that emit useful light.

SUMMARY OF THE INVENTION

The invention provides an electroluminescent device comprising alight-emitting layer including a light emitting material that containsan organometallic complex comprising (1) a metal selected from the groupconsisting of Ir, Rh, Os, Pt, and Pd and (2) a diazole group ligandwherein the ligand has a fused aromatic ring group including a nitrogenof the diazole as a bridgehead nitrogen. The invention also provides adisplay or area lighting device employing the electroluminescent deviceand a process for emitting light using the device.

The device emits light through phosphorescent emission.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-section of a typical OLED device in which thisinvention may be used.

DETAILED DESCRIPTION OF THE INVENTION

The invention is summarized above. The organometallic complex comprisesa metal selected from the group consisting of Ir, Rh, Os, Pt, and Pd anda diazole group ligand wherein the ligand has a fused aromatic ringgroup including a nitrogen of the diazole as a bridgehead nitrogen.

A diazole compound has a five-membered unsaturated ring containing twonitrogen atoms. Suitably the diazole ring is fused with at least onearomatic ring. It is desirable that one of the nitrogen atoms in thediazole ring be a bridgehead nitrogen at the junction with the fusedring. It is further desirable that the diazole ring be furthersubstituted with a substituent that has at least one double bond. Evenmore desirable, the double bond is part of a five or six-memberedaromatic ring. Suitably the double bond forms an additional bond to themetal. Illustrative examples of diazole compounds are shown below.

In one desirable embodiment, the organometallic complex can berepresented by Formula (1).

In Formula (1), Z represents the atoms necessary to complete a diazolering group that is fused with at least one aromatic ring group. Forexample, Z may represent the atoms necessary to complete animidazopyridine ring group or a pyrazolopyridine ring group.

N^(f) represents a nitrogen atom at a bridgehead position between thediazole ring group and the fused aromatic ring group. In one suitableembodiment, N^(f) is not adjacent to the other nitrogen of the diazolering.

M is a coordinated metal selected from the group consisting of Ir, Rh,Os, Pt, and Pd. In one suitable embodiment, M represents Ir or Rh, andmore desirably M represents Ir.

m is 1, 2 or 3 when M is Ir or Rh and m is 1 or 2 when M is Pt, Pd, orOs.

w is 0-4 as necessary in order to satisfy a total of 6 coordinationsites when M is Ir, Os, or Rh, and w is 0-2 as necessary in order tosatisfy 4 coordination sites when M is Pt or Pd.

L represents an independently selected ligand group. The ligand ischosen so that it can complex to the metal. For example, L can representcyanide or a halogen, such as chloride. Desirably, L represents abidentate ligand. For example, L can represent a carbonyl-substitutedgroup such as an acetylacetonate group, a hexafluoroacetylacetonategroup, or a salicylaldehyde group. L can also represent an acidderivative of formula R^(a)CO₂ ⁻, wherein R^(a) represents an arylgroup, such as phenyl or tolyl group, or an alkyl group, such as methylor butyl group. Examples of desirable acid derivatives are picolinicacid and 3-bromopicolinic acid. L can also represent an alkoxide offormula R^(a)O—, wherein R^(a) is as defined above, for example, R^(a)O—can represent a phenoxide group.

One useful embodiment of the organometallic complex of Formula (1) isrepresented by Formula (1a):

In Formula (1a), M, L, w, m, R¹, R² are as defined previously.

R³ represents hydrogen or a substituent. Examples of suitablesubstituents are groups such as an alkyl group, for example a methylgroup or a trifluoromethyl group and an aryl group such as a phenylgroup, or tolyl group.

Ar represents the atoms necessary to form an aromatic ring group, suchas a pyridine ring group or pyrrole ring group, a quinoline ring group,an isoquinoline ring group, a pyrimidine ring group, a pyrazine ringgroup, or a triazine ring group, and Ar may have additional fused rings.

In another useful embodiment of the invention the organometallic complexof Formula (1) is represented by Formula (1b):

In Formula (1b), M, L, w, m, R¹, R², R³, and Ar are as definedpreviously.

In another suitable embodiment of the invention the organometalliccomplex of Formula (1) is represented by Formula (1c):

In Formula (1c), M, L, w, m, R¹, R², R³, and Ar are as definedpreviously.

In another suitable embodiment of the invention the organometalliccomplex of Formula (1) is represented by Formula (1d):

In Formula (1d), M, L, w, m, R¹, R², R³ and Ar are defined previously.

R¹ and R² represent substituent groups, provided that R¹ and R² may forma ring. Examples of R¹ and R² include aromatic groups such as phenyl,and tolyl groups and alkyl groups such as methyl and ethyl groups. Inone desirable embodiment, R¹ and R² represent atoms that join togetherto form an aromatic ring, such as a benzene ring group. Illustrativeexamples of R¹ and R² are shown below. R¹ and R² may be chosen so as tofuse with the diazole ring group.

In one preferred embodiment, the organometallic compound of theinvention is represented by Formula (1), wherein: w is 0, m is 3, and Mrepresents Ir, and R¹ and R² represent the atoms necessary to form asix-membered aromatic ring group.

Suitably, the light-emitting layer of the device comprises a host andone or more dopants where the dopant(s) is present in an amount of up to15 wt % of the host, more typically from 0. 1-10.0 wt % and often from2.0-10.0 wt % of the host. At least one dopant is suitably a complexcomprising a ring system of Formula (1).

Desirable hosts are capable of forming a continuous film and the host isselected so as to have a high level of energy transfer from the host tothe dopant material. Examples of desirable hosts are4,4′-N,N′-dicarbazole-biphenyl (CBP),2,2′-dimethyl-4,4′-N,N′-dicarbazole-biphenyl, and1,3-(N,N′-dicarbazole)benzene.

In one useful embodiment the multilayer electroluminescent devicecontains a hole-blocking layer located between the light-emitting layerand the electron-transporting layer and preferably adjacent to thelight-emitting layer. This layer can increase the efficiency of thedevice. Examples of useful hole-blocking materials arebis(2-methyl-quinolinolate)(4-phenylphenolate)Al (Balq) and2,9-dimethyl-4,7-diphenyl-phenanthroline (bathocuproine, BCP).

In another useful embodiment of the invention the organometallic complexof Formula (1) is represented by Formula (1e):

In Formula (1e), Ar and R³ are defined previously.

Ar² represents the atoms necessary to form a five or six memberedaromatic ring group, such as benzene group and naphthalene group. In onedesirable embodiment, Ar and Ar² represent the atoms necessary to form asix-membered aromatic ring group, which may include additional fusedrings.

In another useful embodiment of the invention the organometallic complexof Formula (1) is represented by Formula (1f):

In Formula (1f), R³, Ar and Ar² are as defined previously. In onedesirable embodiment, Ar and Ar² represent the atoms necessary to formsix-membered aromatic ring groups which may include additional fusedrings.

In another useful embodiment of the invention the organometallic complexof Formula (1) is represented by Formula (1g):

In Formula (Ig), R³, Ar and Ar² are as defined previously. In onedesirable embodiment, Ar and Ar² represent the atoms necessary to formsix-membered aromatic ring groups, which may include additional fusedrings.

In another useful embodiment of the invention the organometallic complexof Formula (1) is represented by Formula (1h):

In Formula (1h), R³, Ar and Ar²are as defined previously. In onedesirable embodiment, Ar and Ar² represent the atoms necessary to formsix-membered aromatic ring groups which may include additional fusedrings.

In one useful embodiment, at least one layer of the EL device, such as aLEL layer, comprises polymeric material. In another suitable embodiment,at least two layers of the OLED device comprise polymeric material.

In one desirable embodiment the EL device is part of a display device.In another suitable embodiment the EL device is part of an area lightingdevice.

Embodiments of the invention can provide advantageous features such asoperating efficiency, higher luminance, color hue, low drive voltage,and improved operating stability. Embodiments of the organometalliccompounds useful in the invention can provide a wide range of huesincluding those useful in the emission of white light (directly orthrough filters).

In many cases the hue of an organometallic light-emitting compounduseful in the invention can be estimated by calculating its tripletstate energy. The triplet state energy for a molecule is defined as thedifference between the ground state energy (E(gs)) of the molecule andthe energy of the lowest triplet state (E(ts)) of the molecule, bothgiven in eV. These energies can be calculated using the B3LYP method asimplemented in the Gaussian98 (Gaussian, Inc., Pittsburgh, Pa.) computerprogram. The basis set for use with the B3LYP method is defined asfollows: MIDI! for all atoms for which MIDI! is defined, 6-31G* for allatoms defined in 6-31G* but not in MIDI!, and either the LACV3P or theLANL2DZ basis set and pseudopotential for atoms not defined in MIDI! or6-31G*, with LACV3P being the preferred method. For any remaining atoms,any published basis set and pseudopotential may be used. MIDI!, 6-31G*and LANL2DZ are used as implemented in the Gaussian98 computer code andLACV3P is used as implemented in the Jaguar 4.1 (Schrodinger, Inc.,Portland Oreg.) computer code. The energy of each state is computed atthe minimum-energy geometry for that state. The difference in energybetween the two states is further modified by Equation 1 to give thetriplet state energy (E(t)):E(t)=0.84*(E(ts)−E(gs))+0.35.

For polymeric or oligomeric materials, it is sufficient to compute thetriplet energy over a monomer or oligomer of sufficient size so thatadditional units do not substantially change the computed tripletenergy.

Synthesis of the emitting materials useful in the invention may beaccomplished by preparing the organic ligand and then using a metal tocomplex the ligand and form the organometallic compound. The synthesisof ligands useful in the invention may be accomplished by variousmethods found in the literature, for example, see T. Benincori, E.Brenna, F. Sannicolo, J. Chem. Soc., Perkin Trans. 1, 675 (1993), R.Tachikawa, S. Tanaka, A. Terada, Atsusuke, Heterocycles, 15, 369 (1981),C. Enguehard, J. Renou, V. Collot, M. Hervet, S. Rault, A Gueiffier, J.of Org. Chem., 65, 6572 (2000) and R. Adams, J. Dix, J. Amer, Chem.Soc., 80 46 (1958).

The organometallic complex useful in the invention may be synthesizedfrom the prepared ligands by various literature methods. For example,see A. Tamayo, B. Alleyne, P. Djurovich, S. Lamansky, I. Tsyba, N. Ho,R. Bau, M. Thompson, J. Amer. Chem. Soc. 125, 7377 (2003), H. Konno,Y.Sasaki, Chem. Lett., 32, 252 (2003), and V. Grushin, N. Hurron, D.LeCloux, W. Marshall, V. Petrov, and Y. Wang, Chem. Comm., 1494 (2001).

Illustrative examples of complexes of Formula (1) useful in the presentinvention are the following:

Unless otherwise specifically stated, use of the term “substituted” or“substituent” means any group or atom other than hydrogen. Unlessotherwise provided, when a group (including a compound or complex)containing a substitutable hydrogen is referred to, it is also intendedto encompass not only the unsubstituted form, but also form furthersubstituted derivatives with any substituent group or groups as hereinmentioned, so long as the substituent does not destroy propertiesnecessary for utility. Suitably, a substituent group may be halogen ormay be bonded to the remainder of the molecule by an atom of carbon,silicon, oxygen, nitrogen, phosphorous, sulfur, selenium, or boron. Thesubstituent may be, for example, halogen, such as chloro, bromo orfluoro; nitro; hydroxyl; cyano; carboxyl; or groups which may be furthersubstituted, such as alkyl, including straight or branched chain orcyclic alkyl, such as methyl, trifluoromethyl, ethyl, t-butyl,3-(2,4-di-t-pentylphenoxy) propyl, and tetradecyl; alkenyl, such asethylene, 2-butene; alkoxy, such as methoxy, ethoxy, propoxy, butoxy,2-methoxyethoxy, sec-butoxy, hexyloxy, 2-ethylhexyloxy, tetradecyloxy,2-(2,4-di-t-pentylphenoxy)ethoxy, and 2-dodecyloxyethoxy; aryl such asphenyl, 4-t-butylphenyl, 2,4,6-trimethylphenyl, naphthyl; aryloxy, suchas phenoxy, 2-methylphenoxy, alpha- or beta-naphthyloxy, and 4-tolyloxy;carbonamido, such as acetamido, benzamido, butyramido, tetradecanamido,alpha-(2,4-di-t-pentyl-phenoxy)acetamido,alpha-(2,4-di-t-pentylphenoxy)butyramido,alpha-(3-pentadecylphenoxy)-hexanamido,alpha-(4-hydroxy-3-t-butylphenoxy)-tetradecanamido,2-oxo-pyrrolidin-1-yl, 2-oxo-5-tetradecylpyrrolin-1-yl,N-methyltetradecanamido, N-succinimido, N-phthalimido,2,5-dioxo-1-oxazolidinyl, 3-dodecyl-2,5-dioxo-1-imidazolyl, andN-acetyl-N-dodecylamino, ethoxycarbonylamino, phenoxycarbonylamino,benzyloxycarbonylamino, hexadecyloxycarbonylamino,2,4-di-t-butylphenoxycarbonylamino, phenylcarbonylamino,2,5-(di-t-pentylphenyl)carbonylamino, p-dodecyl-phenylcarbonylamino,p-tolylcarbonylamino, N-methylureido, N,N-dimethylureido,N-methyl-N-dodecylureido, N-hexadecylureido, N,N-dioctadecylureido,N,N-dioctyl-N′-ethylureido, N-phenylureido, N,N-diphenylureido,N-phenyl-N-p-tolylureido, N-(m-hexadecylphenyl)ureido,N,N-(2,5-di-t-pentylphenyl)-N′-ethylureido, and t-butylcarbonamido;sulfonamido, such as methylsulfonamido, enzenesulfonamido,p-tolylsulfonamido, p-dodecylbenzenesulfonamido,N-methyltetradecylsulfonamido, N,N-dipropyl-sulfamoylamino, andhexadecylsulfonamido; sulfamoyl, such as N-methylsulfamoyl,N-ethylsulfamoyl, N,N-dipropylsulfamoyl, N-hexadecylsulfamoyl,N,N-dimethylsulfamoyl, N-[3-(dodecyloxy)propyl]sulfamoyl,N-[4-(2,4-di-t-pentylphenoxy)butyl]sulfamoyl,N-methyl-N-tetradecylsulfamoyl, and N-dodecylsulfamoyl; carbamoyl, suchas N-methylcarbamoyl, N,N-dibutylcarbamoyl, N-octadecylcarbamoyl,N-[4-(2,4-di-t-pentylphenoxy)butyl]carbamoyl,N-methyl-N-tetradecylcarbamoyl, and N,N-dioctylcarbamoyl; acyl, such asacetyl, (2,4-di-t-amylphenoxy)acetyl, phenoxycarbonyl,p-dodecyloxyphenoxycarbonyl methoxycarbonyl, butoxycarbonyl,tetradecyloxycarbonyl, ethoxycarbonyl, benzyloxycarbonyl,3-pentadecyloxycarbonyl, and dodecyloxycarbonyl; sulfonyl, such asmethoxysulfonyl, octyloxysulfonyl, tetradecyloxysulfonyl,2-ethylhexyloxysulfonyl, phenoxysulfonyl,2,4-di-t-pentylphenoxysulfonyl, methylsulfonyl, octylsulfonyl,2-ethylhexylsulfonyl, dodecylsulfonyl, hexadecylsulfonyl,phenylsulfonyl, 4-nonylphenylsulfonyl, and p-tolylsulfonyl; sulfonyloxy,such as dodecylsulfonyloxy, and hexadecylsulfonyloxy; sulfinyl, such asmethylsulfinyl, octylsulfinyl, 2-ethylhexylsulfinyl, dodecylsulfinyl,hexadecylsulfinyl, phenylsulfinyl, 4-nonylphenylsulfinyl, andp-tolylsulfinyl; thio, such as ethylthio, octylthio, benzylthio,tetradecylthio, 2-(2,4-di-t-pentylphenoxy)ethylthio, phenylthio,2-butoxy-5-t-octylphenylthio, and p-tolylthio; acyloxy, such asacetyloxy, benzoyloxy, octadecanoyloxy, p-dodecylaniidobenzoyloxy,N-phenylcarbaamoyloxy, N-ethylcarbaamoyloxy, and cyclohexylcarbonyloxy;amine, such as phenylanilino, 2-chloroanilino, diethylamine,dodecylamine; imino, such as 1 (N-phenylimido)ethyl, N-succinimido or3-benzylhydantoinyl; phosphate, such as dimethylphosphate andethylbutylphosphate; phosphite, such as diethyl and dihexylphosphite; aheterocyclic group, a heterocyclic oxy group or a heterocyclic thiogroup, each of which may be substituted and which contain a 3 to 7membered heterocyclic ring composed of carbon atoms and at least onehetero atom selected from the group consisting of oxygen, nitrogen,sulfur, phosphorous, or boron. such as 2-furyl, 2-thienyl,2-benzimidazolyloxy or 2-benzothiazolyl; quaternary ammonium, such astriethylammonium; quaternary phosphonium, such as triphenylphosphonium;and silyloxy, such as trimethylsilyloxy.

If desired, the substituents may themselves be further substituted oneor more times with the described substituent groups. The particularsubstituents used may be selected by those skilled in the art to attainthe desired desirable properties for a specific application and caninclude, for example, electron-withdrawing groups, electron-donatinggroups, and steric groups. When a molecule may have two or moresubstituents, the substituents may be joined together to form a ringsuch as a fused ring unless otherwise provided. Generally, the abovegroups and substituents thereof may include those having up to 48 carbonatoms, typically 1 to 36 carbon atoms and usually less than 24 carbonatoms, but greater numbers are possible depending on the particularsubstituents selected.

Suitably, the light-emitting layer of the OLED device comprises a hostmaterial and one or more guest materials for emitting light. At leastone of the guest materials is suitably a phosphorescent complexcomprising a ring system of Formula 1. The light-emitting guestmaterial(s) is usually present in an amount less than the amount of hostmaterials and is typically present in an amount of up to 15 wt % of thehost, more typically from 0.1-10.0 wt % of the host. For convenience,the phosphorescent complex guest material may be referred to herein as aphosphorescent material. The phosphorescent material of Formula 1 ispreferably a low molecular weight compound, but it may also be anoligomer or a polymer having a main chain or a side chain of repeatingunits having the moiety represented by Formula 1. It may be provided asa discrete material dispersed in the host material, or it may be bondedin some way to the host material, for example, covalently bonded into apolymeric host.

Host Materials for Phosphorescent Materials

Suitable host materials should be selected so that the triplet excitoncan be transferred efficiently from the host material to thephosphorescent material. For this transfer to occur, it is a highlydesirable condition that the excited state energy of the phosphorescentmaterial be lower than the difference in energy between the lowesttriplet state and the ground state of the host. However, the band gap ofthe host should not be chosen so large as to cause an unacceptableincrease in the drive voltage of the OLED. Suitable host materials aredescribed in WO 00/70655 A2; 01/39234 A2; 01/93642 A1; 02/074015 A2;02/15645 A1, and U.S. 2002/0117662. Suitable hosts include certain arylamines, triazoles, indoles and carbazole compounds. Examples ofdesirable hosts are 4,4′-N,N′-dicarbazole-biphenyl (CBP),2,2′-dimethyl-4,4′-N,N′-dicarbazole-biphenyl,m-(N,N′-dicarbazole)benzene, and poly(N-vinylcarbazole), including theirderivatives.

Desirable host materials are capable of forming a continuous film. Thelight-emitting layer may contain more than one host material in order toimprove the device's film morphology, electrical properties, lightemission efficiency, and lifetime. The light emitting layer may containa first host material that has good hole-transporting properties, and asecond host material that has good electron-transporting properties.

Other Phosphorescent Materials

Phosphorescent materials of Formula 1 may be used in combination withother phosphorescent materials, either in the same or different layers.Some other phosphorescent materials are described in WO 00/57676, WO00/70655, WO 01/41512 A1, WO 02/15645 A1, U.S. 2003/0017361 A1, WO01/93642 A1, WO 01/39234 A2, U.S. 6,458,475 B1, WO 02/071813 A1, U.S.Pat. No. 6,573,651 B2, U.S. 2002/0197511 A1, WO 02/074015 A2, U.S.6,451,455 B1, U.S. 2003/0072964 A1, U.S. 2003/0068528 A1, U.S. Pat. No.6,413,656 B1, U.S. Pat. No.6,515,298 B2, U.S. Pat. No. 6,451,415 B1,U.S. Pat. No. 6,097,147, U.S. 2003/0124381 A1, U.S. 2003/0059646 A1,U.S. 2003/0054198 A1, EP 1 239 526 A2, EP 1 238 981 A2, EP 1 244 155 A2,U.S. 2002/0100906 A1, U.S. 2003/0068526 A1, U.S. 2003/0068535 A1, JP2003073387A, JP 2003 073388A, U.S. 2003/0141809 A1, U.S. 2003/0040627A1, JP 2003059667A, JP 2003073665A, and U.S. 2002/0121638 A1.

The emission wavelengths of cyclometallated Ir(III) complexes of thetype IrL₃ and IrL₂L′, such as thegreen-emittingfac-tris(2-phenylpyridinato-N,C^(2′))Iridium(III) andbis(2-phenylpyridinato-N,C^(2′))Iridium(III)(acetylacetonate) may beshifted by substitution of electron donating or withdrawing groups atappropriate positions on the cyclometallating ligand L, or by choice ofdifferent heterocycles for the cyclometallating ligand L. The emissionwavelengths may also be shifted by choice of the ancillary ligand L′.Examples of red emitters are thebis(2-(2′-benzothienyl)pyridinato-N,C^(3′))Iridium(III)(acetylacetonate)and tris(1-phenylisoquinolinato-N,C^(2′))Iridium(III). A blue-emittingexample isbis(2-(4,6-diflourophenyl)-pyridinato-N,C^(2′))Iridium(III)(picolinate).

Red electrophosphorescence has been reported, usingbis(2-(2′-benzo[4,5-a]thienyl)pyridinato-N,C³) iridium (acetylacetonate)[Btp₂Ir(acac)] as the phosphorescent material (Adachi, C., Lamansky, S.,Baldo, M. A., Kwong, R. C., Thompson, M. E., and Forrest, S. R., App.Phys. Lett., 78, 1622-1624 (2001).

Other important phosphorescent materials include cyclometallated Pt(II)complexes such as cis-bis(2-phenylpyridinato-N,C^(2′))platinum(II),cis-bis(2-(2′-thienyl)pyridinato-N,C^(3′)) platinum(II),cis-bis(2-(2′-thienyl)quinolinato-N,C^(5′)) platinum(II), or(2-(4,6-diflourophenyl)pyridinato-NC^(2′)) platinum (II)acetylacetonate. Pt(II) porphyrin complexes such as2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine platinum(II) are alsouseful phosphorescent materials.

Still other examples of useful phosphorescent materials includecoordination complexes of the trivalent lanthanides such as Tb³⁺ andEu³⁺ (J. Kido et al, Appl. Phys. Lett., 65, 2124 (1994))

Blocking Layers

In addition to suitable hosts, an OLED device employing a phosphorescentmaterial often requires at least one exciton- or hole- orelectron-blocking layer to help confine the excitons or electron-holerecombination centers to the light-emitting layer comprising the hostand phosphorescent material. In one embodiment, such a blocking layerwould be placed between the electron-transporting layer and thelight-emitting layer—see FIG. 1, layer 110. In this case, the ionizationpotential of the blocking layer should be such that there is an energybarrier for hole migration from the host into the electron-transportinglayer, while the electron affinity should be such that electrons passmore readily from the electron-transporting layer into thelight-emitting layer comprising host and phosphorescent material. It isfurther desired, but not absolutely required, that the triplet energy ofthe blocking material be greater than that of the phosphorescentmaterial. Suitable hole-blocking materials are described in WO00/70655A2 and WO 01/93642 A1. Two examples of useful materials arebathocuproine (BCP) andbis(2-methyl-8-quinolinolato)(4-phenylphenolato)Aluminum(III) (BAlQ).Metal complexes other than Balq are also known to block holes andexcitons as described in U.S. 2003/0068528. U.S. 2003/0175553 A1describes the use of fac-tris(1-phenylpyrazolato-N,C^(2′))iridium(II)(Irppz) in an electron/exciton blocking layer.

Embodiments of the invention can provide advantageous features such asoperating efficiency, higher luminance, color hue, low drive voltage,and improved operating stability. Embodiments of the organometalliccompounds useful in the invention can provide a wide range of huesincluding those useful in the emission of white light (directly orthrough filters to provide multicolor displays).

General Device Architecture

The present invention can be employed in many OLED device configurationsusing small molecule materials, oligomeric materials, polymericmaterials, or combinations thereof These include very simple structurescomprising a single anode and cathode to more complex devices, such aspassive matrix displays comprised of orthogonal arrays of anodes andcathodes to form pixels, and active-matrix displays where each pixel iscontrolled independently, for example, with thin film transistors(TFTs).

There are numerous configurations of the organic layers wherein thepresent invention can be successfully practiced. The essentialrequirements of an OLED are an anode, a cathode, and an organiclight-emitting layer located between the anode and cathode. Additionallayers may be employed as more fully described hereafter.

A typical structure, especially useful for of a small molecule device,is shown in FIG. 1 and is comprised of a substrate 101, an anode 103, ahole-injecting layer 105, a hole-transporting layer 107, alight-emitting layer 109, a hole- or exciton-blocking layer 110, anelectron-transporting layer 111, and a cathode 113. These layers aredescribed in detail below. Note that the substrate may alternatively belocated adjacent to the cathode, or the substrate may actuallyconstitute the anode or cathode. The organic layers between the anodeand cathode are conveniently referred to as the organic EL element.Also, the total combined thickness of the organic layers is desirablyless than 500 nm.

The anode and cathode of the OLED are connected to a voltage/currentsource through electrical conductors. The OLED is operated by applying apotential between the anode and cathode such that the anode is at a morepositive potential than the cathode. Holes are injected into the organicEL element from the anode and electrons are injected into the organic ELelement at the cathode. Enhanced device stability can sometimes beachieved when the OLED is operated in an AC mode where, for some timeperiod in the cycle, the potential bias is reversed and no currentflows. An example of an AC driven OLED is described in U.S. Pat. No.5,552,678.

Substrate

The OLED device of this invention is typically provided over asupporting substrate 101 where either the cathode or anode can be incontact with the substrate. The electrode in contact with the substrateis conveniently referred to as the bottom electrode. Conventionally, thebottom electrode is the anode, but this invention is not limited to thatconfiguration. The substrate can either be light transmissive or opaque,depending on the intended direction of light emission. The lighttransmissive property is desirable for viewing the EL emission throughthe substrate. Transparent glass or plastic is commonly employed in suchcases. The substrate can be a complex structure comprising multiplelayers of materials. This is typically the case for active matrixsubstrates wherein TFTs are provided below the OLED layers. It is stillnecessary that the substrate, at least in the emissive pixilated areas,be comprised of largely transparent materials such as glass or polymers.For applications where the EL emission is viewed through the topelectrode, the transmissive characteristic of the bottom support isimmaterial, and therefore can be light transmissive, light absorbing orlight reflective. Substrates for use in this case include, but are notlimited to, glass, plastic, semiconductor materials, silicon, ceramics,and circuit board materials. Again, the substrate can be a complexstructure comprising multiple layers of materials such as found inactive matrix TFT designs. It is necessary to provide in these deviceconfigurations a light-transparent top electrode.

Anode

When the desired electroluminescent light emission (EL) is viewedthrough the anode, the anode should be transparent or substantiallytransparent to the emission of interest. Common transparent anodematerials used in this invention are indium-tin oxide (ITO), indium-zincoxide (IZO) and tin oxide, but other metal oxides can work including,but not limited to, aluminum- or indium-doped zinc oxide,magnesium-indium oxide, and nickel-tungsten oxide. In addition to theseoxides, metal nitrides, such as gallium nitride, and metal selenides,such as zinc selenide, and metal sulfides, such as zinc sulfide, can beused as the anode. For applications where EL emission is viewed onlythrough the cathode, the transmissive characteristics of the anode areimmaterial and any conductive material can be used, transparent, opaqueor reflective. Example conductors for this application include, but arenot limited to, gold, iridium, molybdenum, palladium, and platinum.Typical anode materials, transmissive or otherwise, have a work functionof 4.1 eV or greater. Desired anode materials are commonly deposited byany suitable means such as evaporation, sputtering, chemical vapordeposition, or electrochemical means. Anodes can be patterned usingwell-known photolithographic processes. Optionally, anodes may bepolished prior to application of other layers to reduce surfaceroughness so as to minimize shorts or enhance reflectivity.

Cathode

When light emission is viewed solely through the anode, the cathode usedin this invention can be comprised of nearly any conductive material.Desirable materials have good film-forming properties to ensure goodcontact with the underlying organic layer, promote electron injection atlow voltage, and have good stability. Useful cathode materials oftencontain a low work function metal (<4.0 eV) or metal alloy. One usefulcathode material is comprised of a Mg:Ag alloy wherein the percentage ofsilver is in the range of 1 to 20%, as described in U.S. Pat. No.4,885,221. Another suitable class of cathode materials includes bilayerscomprising the cathode and a thin electron-injection layer (EIL) incontact with an organic layer (e.g., an electron transporting layer(ETL)) which is capped with a thicker layer of a conductive metal. Here,the EIL preferably includes a low work function metal or metal salt, andif so, the thicker capping layer does not need to have a low workfunction. One such cathode is comprised of a thin layer of LiF followedby a thicker layer of Al as described in U.S. Pat. No. 5,677,572. An ETLmaterial doped with an alkali metal, for example, Li-doped Alq, isanother example of a useful EIL. Other useful cathode material setsinclude, but are not limited to, those disclosed in U.S. Pat. Nos.5,059,861, 5,059,862, and 6,140,763.

When light emission is viewed through the cathode, the cathode must betransparent or nearly transparent. For such applications, metals must bethin or one must use transparent conductive oxides, or a combination ofthese materials. Optically transparent cathodes have been described inmore detail in U.S. Pat. No. 4,885,211, U.S. Pat. No. 5,247,190, JP3,234,963, U.S. Pat. No. 5,703,436, U.S. Pat. No. 5,608,287, U.S. Pat.No. 5,837,391, U.S. Pat. No. 5,677,572, U.S. Pat. No. 5,776,622, U.S.Pat. No. 5,776,623, U.S. Pat. No. 5,714,838, U.S. Pat. No. 5,969,474,U.S. Pat. No. 5,739,545, U.S. Pat. No. 5,981,306, U.S. Pat. No.6,137,223, U.S. Pat. No. 6,140,763, U.S. Pat. No. 6,172,459, EP 1 076368, U.S. Pat. No. 6,278,236, and U.S. Pat. No. 6,284,3936. Cathodematerials are typically deposited by any suitable method such asevaporation, sputtering, or chemical vapor deposition. When needed,patterning can be achieved through many well known methods including,but not limited to, through-mask deposition, integral shadow masking asdescribed in U.S. Pat. No. 5,276,380 and EP 0 732 868, laser ablation,and selective chemical vapor deposition.

Hole-Injecting Layer (HIL)

A hole-injecting layer 105 may be provided between anode 103 andhole-transporting layer 107. The hole-injecting material can serve toimprove the film formation property of subsequent organic layers and tofacilitate injection of holes into the hole-transporting layer. Suitablematerials for use in the hole-injecting layer include, but are notlimited to, porphyrinic compounds as described in U.S. Pat. No.4,720,432, plasma-deposited fluorocarbon polymers as described in U.S.Pat. No. 6,208,075, and some aromatic amines, for example, m-MTDATA(4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine). Alternativehole-injecting materials reportedly useful in organic EL devices aredescribed in EP 0 891 121 A1 and EP 1 029 909 A1.

Hole-Transporting Layer (HTL)

The hole-transporting layer 107 of the organic EL device contains atleast one hole-transporting compound such as an aromatic tertiary amine,where the latter is understood to be a compound containing at least onetrivalent nitrogen atom that is bonded only to carbon atoms, at leastone of which is a member of an aromatic ring. In one form the aromatictertiary amine can be an arylamine, such as a monoarylamine,diarylamine, triarylamine, or a polymeric arylamine. Exemplary monomerictriarylamines are illustrated by Klupfel et al. in U.S. Pat. No.3,180,730. Other suitable triarylamines substituted with one or morevinyl radicals and/or comprising at least one active hydrogen containinggroup are disclosed by Brantley et al. in U.S. Pat. No. 3,567,450 andU.S. Pat. No. 3,658,520.

A more preferred class of aromatic tertiary amines are those whichinclude at least two aromatic tertiary amine moieties as described inU.S. Pat. No. 4,720,432 and U.S. Pat. No. 5,061,569. Such compoundsinclude those represented by structural formula (A).

wherein Q₁ and Q₂ are independently selected aromatic tertiary aminemoieties and G is a linking group such as an arylene, cycloalkylene, oralkylene group of a carbon to carbon bond. In one embodiment, at leastone of Q₁ or Q₂ contains a polycyclic fused ring structure, e.g., anaphthalene. When G is an aryl group, it is conveniently a phenylene,biphenylene, or naphthalene moiety.

A useful class of triarylamines satisfying structural formula (A) andcontaining two triarylamine moieties is represented by structuralformula (B):

where

-   -   R₁ and R₂ each independently represents a hydrogen atom, an aryl        group, or an alkyl group or R₁ and R₂ together represent the        atoms completing a cycloalkyl group; and    -   R₃ and R₄ each independently represents an aryl group, which is        in turn substituted with a diaryl substituted amino group, as        indicated by structural formula (C):        wherein R₅ and R₆ are independently selected aryl groups. In one        embodiment, at least one of R₅ or R₆ contains a polycyclic fused        ring structure, e.g., a naphthalene.

Another class of aromatic tertiary amines are the tetraaryldiamines.Desirable tetraaryldiamines include two diarylamino groups, such asindicated by formula (C), linked through an arylene group. Usefultetraaryldiamines include those represented by formula (D).

wherein

-   -   each Are is an independently selected arylene group, such as a        phenylene or anthracene moiety,    -   n is an integer of from 1 to 4, and    -   Ar, R₇, R₈, and R₉ are independently selected aryl groups.

In a typical embodiment, at least one of Ar, R₇, R₈, and R₉ is apolycyclic fised ring structure, e.g., a naphthalene

The various alkyl, alkylene, aryl, and arylene moieties of the foregoingstructural formulae (A), (B), (C), (D), can each in turn be substituted.Typical substituents include alkyl groups, alkoxy groups, aryl groups,aryloxy groups, and halogen such as fluoride, chloride, and bromide. Thevarious alkyl and alkylene moieties typically contain from about 1 to 6carbon atoms. The cycloalkyl moieties can contain from 3 to about 10carbon atoms, but typically contain five, six, or seven ring carbonatoms—e.g., cyclopentyl, cyclohexyl, and cycloheptyl ring structures.The aryl and arylene moieties are usually phenyl and phenylene moieties.

The hole-transporting layer can be formed of a single or a mixture ofaromatic tertiary amine compounds. Specifically, one may employ atriarylamine, such as a triarylamine satisfying the formula (B), incombination with a tetraaryldiamine, such as indicated by formula (D).When a triarylamine is employed in combination with a tetraaryldiamine,the latter is positioned as a layer interposed between the triarylamineand the electron injecting and transporting layer. Illustrative ofuseful aromatic tertiary amines are the following:

-   -   1,1 -Bis(4-di-p-tolylaminophenyl)cyclohexane    -   1,1 -Bis(4-di-p-tolylaminophenyl)-4-phenylcyclohexane    -   N,N,N′,N′-tetraphenyl-4,4′″-diamino-1,1′:4′,1″:4″,1′″-quaterphenyl    -   Bis(4-dimethylamino-2-methylphenyl)phenylmethane    -   1,4-bis[2-[4-[N,N-di(p-toly)amino]phenyl]vinyl]benzene (BDTAPVB)    -   N,N,N′,N′-Tetra-p-tolyl-4,4′-diaminobiphenyl    -   N,N,N′,N′-Tetraphenyl-4,4′-diaminobiphenyl    -   N,N,N′,N′-tetra-1-naphthyl-4,4′-diaminobiphenyl    -   N,N,N′,N′-tetra-2-naphthyl-4,4′-diaminobiphenyl    -   N-Phenylcarbazole    -   4,4′-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB)    -   4,4′-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]biphenyl (TNB)    -   4,4′-Bis[N-(1-naphthyl)-N-phenylamino]p-terphenyl    -   4,4′-Bis[N-(2-naphthyl)-N-phenylamino]biphenyl    -   4,4′-Bis[N-(3-acenaphthenyl)-N-phenylamino]biphenyl    -   1,5-Bis[N-(1-naphthyl)-N-phenylamino]naphthalene    -   4,4′-Bis[N-(9-anthryl)-N-phenylamino]biphenyl    -   4,4′-Bis[N-(1-anthryl)-N-phenylamino]-p-terphenyl    -   4,4′-Bis[N-(2-phenanthryl)-N-phenylamino]biphenyl    -   4,4′-Bis[N-(8-fluoranthenyl)-N-phenylamino]biphenyl    -   4,4′-Bis[N-(2-pyrenyl)-N-phenylamino]biphenyl    -   4,4′-Bis[N-(2-naphthacenyl)-N-phenylamino]biphenyl    -   4,4′-Bis[N-(2-perylenyl)-N-phenylamino]biphenyl    -   4,4′-Bis[N-(1-coronenyl)-N-phenylamino]biphenyl    -   2,6-Bis(di-p-tolylamino)naphthalene    -   2,6-Bis[di-(1-naphthyl)amino]naphthalene    -   2,6-Bis[N-(1-naphthyl)-N-(2-naphthyl)amino]naphthalene    -   N,N,N′,N′-Tetra(2-naphthyl)-4,4″-diamino-p-terphenyl    -   4,4′-Bis {N-phenyl-N-[4-(1-naphthyl)-phenyl]amino}biphenyl    -   2,6-Bis[N,N-di(2-naphthyl)amino]fluorene    -   4,4′,4″-tris[(3-methylphenyl)phenylamino]triphenylamine (MTDATA)    -   4,4′-Bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (TPD)

Another class of useful hole-transporting materials includes polycyclicaromatic compounds as described in EP 1 009 041. Tertiary aromaticamines with more than two amine groups may be used including oligomericmaterials. In addition, polymeric hole-transporting materials can beused such as poly(N-vinylcarbazole) (PVK), polythiophenes, polypyrrole,polyaniline, and copolymers such aspoly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) also calledPEDOT/PSS.

Fluorescent Light-Emitting Materials and Layers (LEL)

In addition to the phosphorescent materials of this invention, otherlight emitting materials may be used in the OLED device, includingfluorescent materials. Although the term “fluorescent” is commonly usedto describe any light emitting material, in this case we are referringto a material that emits light from a singlet excited state. Fluorescentmaterials may be used in the same layer as the phosphorescent material,in adjacent layers, in adjacent pixels, or any combination. Care must betaken not to select materials that will adversely affect the performanceof the phosphorescent materials of this invention. One skilled in theart will understand that triplet excited state energies of materials inthe same layer as the phosphorescent material or in an adjacent layermust be appropriately set so as to prevent unwanted quenching.

As more fully described in U.S. Pat. Nos. 4,769,292 and 5,935,721, thelight-emitting layer (LEL) of the organic EL element includes aluminescent fluorescent or phosphorescent material whereelectroluminescence is produced as a result of electron-hole pairrecombination in this region. The light-emitting layer can be comprisedof a single material, but more commonly consists of a host materialdoped with a guest emitting material or materials where light emissioncomes primarily from the emitting materials and can be of any color. Thehost materials in the light-emitting layer can be anelectron-transporting material, as defined below, a hole-transportingmaterial, as defined above, or another material or combination ofmaterials that support hole-electron recombination. Fluorescent emittingmaterials are typically incorporated at 0.01 to 10% by weight of thehost material.

The host and emitting materials can be small non-polymeric molecules orpolymeric materials such as polyfluorenes and polyvinylarylenes (e.g.,poly(p-phenylenevinylene), PPV). In the case of polymers, small moleculeemitting materials can be molecularly dispersed into a polymeric host,or the emitting materials can be added by copolymerizing a minorconstituent into a host polymer. Host materials may be mixed together inorder to improve film formation, electrical properties, light emissionefficiency, lifetime, or manufacturability. The host may comprise amaterial that has good hole-transporting properties and a material thathas good electron-transporting properties.

An important relationship for choosing a fluorescent dye as a guestemitting material is a comparison of the singlet excited state energiesof the host and light-emitting material. For efficient energy transferfrom the host to the emitting material, a highly desirable condition isthat the singlet excited state energy of the emitting material is lowerthan that of the host material.

Host and emitting materials known to be of use include, but are notlimited to, those disclosed in U.S. Pat. No. 4,768,292, U.S. Pat. No.5,141,671, U.S. Pat. No. 5,150,006, U.S. Pat. No. 5,151,629, U.S. Pat.No. 5,405,709, U.S. Pat. No. 5,484,922, U.S. Pat. No. 5,593,788, U.S.Pat. No. 5,645,948, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,755,999,U.S. Pat. No. 5,928,802, U.S. Pat. No. 5,935,720, U.S. Pat. No.5,935,721, and U.S. Pat. No. 6,020,078.

Metal complexes of 8-hydroxyquinoline and similar derivatives (FormulaE) constitute one class of useful host compounds capable of supportingelectroluminescence, and are particularly suitable for light emission ofwavelengths longer than 500 nm, e.g., green, yellow, orange, and red.

wherein

-   -   M represents a metal;    -   n is an integer of from 1 to 4; and    -   Z independently in each occurrence represents the atoms        completing a nucleus having at least two fused aromatic rings.

From the foregoing it is apparent that the metal can be monovalent,divalent, trivalent, or tetravalent metal. The metal can, for example,be an alkali metal, such as lithium, sodium, or potassium; an alkalineearth metal, such as magnesium or calcium; an earth metal, such aluminumor gallium, or a transition metal such as zinc or zirconium. Generallyany monovalent, divalent, trivalent, or tetravalent metal known to be auseful chelating metal can be employed.

Z completes a heterocyclic nucleus containing at least two fusedaromatic rings, at least one of which is an azole or azine ring.Additional rings, including both aliphatic and aromatic rings, can befused with the two required rings, if required. To avoid addingmolecular bulk without improving on function the number of ring atoms isusually maintained at 18 or less.

Illustrative of useful chelated oxinoid compounds are the following:

-   -   CO-1: Aluminum trisoxine [alias,        tris(8-quinolinolato)aluminum(III)]    -   CO-2: Magnesium bisoxine [alias,        bis(8-quinolinolato)magnesium(II)]    -   CO-3: Bis[benzo{f}-8-quinolinolato]zinc (II)    -   CO-4:        Bis(2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum(III)    -   CO-5: Indium trisoxine [alias, tris(8-quinolinolato)indium]    -   CO-6: Aluminum tris(5-methyloxine) [alias,        tris(5-methyl-8-quinolinolato)aluminum(III)]    -   CO-7: Lithium oxine [alias, (8-quinolinolato)lithium(I)]    -   CO-8: Gallium oxine [alias, tris(8-quinolinolato)gallium(III)]    -   CO-9: Zirconium oxine [alias,        tetra(8-quinolinolato)zirconium(IV)]

Derivatives of 9,10-di-(2-naphthyl)anthracene (Formula F) constitute oneclass of useful host materials capable of supportingelectroluminescence, and are particularly suitable for light emission ofwavelengths longer than 400 nm, e.g., blue, green, yellow, orange orred.

wherein: R¹, R², R³, R⁴, R⁵, and R⁶ represent one or more substituentson each ring where each substituent is individually selected from thefollowing groups:

-   -   Group 1: hydrogen, or alkyl of from 1 to 24 carbon atoms;    -   Group 2: aryl or substituted aryl of from 5 to 20 carbon atoms;    -   Group 3: carbon atoms from 4 to 24 necessary to complete a fused        aromatic ring of anthracenyl; pyrenyl, or perylenyl;    -   Group 4: heteroaryl or substituted heteroaryl of from 5 to 24        carbon atoms as necessary to complete a fused heteroaromatic        ring of furyl, thienyl, pyridyl, quinolinyl or other        heterocyclic systems;    -   Group 5: alkoxylamino, alkylamino, or arylamino of from 1 to 24        carbon atoms; and    -   Group 6: fluorine, chlorine, bromine or cyano.

Illustrative examples include 9,10-di-(2-naphthyl)anthracene and2-t-butyl-9,10-di-(2-naphthyl)anthracene. Other anthracene derivativescan be useful as a host in the LEL, including derivatives of 9,10-bis[4-(2,2-diphenylethenyl)phenyl]anthracene.

Benzazole derivatives (Formula G) constitute another class of usefulhost materials capable of supporting electroluminescence, and areparticularly suitable for light emission of wavelengths longer than 400nm, e.g., blue, green, yellow, orange or red.

Where:

-   -   n is an integer of 3 to 8;    -   Z is 0, NR or S; and    -   R and R′ are individually hydrogen; alkyl of from 1 to 24 carbon        atoms, for example, propyl, t-butyl, heptyl, and the like; aryl        or hetero-atom substituted aryl of from 5 to 20 carbon atoms for        example phenyl and naphthyl, furyl, thienyl, pyridyl, quinolinyl        and other heterocyclic systems; or halo such as chloro, fluoro;        or atoms necessary to complete a fused aromatic ring; and    -   L is a linkage unit consisting of alkyl, aryl, substituted        alkyl, or substituted aryl, which conjugately or unconjugately        connects the multiple benzazoles together. An example of a        useful benzazole is 2,2′,        2″-(1,3,5-phenylene)tris[1-phenyl-1H-benzimidazole].

Styrylarylene derivatives as described in U.S. Pat. No. 5,121,029 and JP08333569 are also useful hosts for blue emission. For example,9,10-bis[4-(2,2-diphenylethenyl)phenyl]anthracene and4,4′-Bis(2,2-diphenylethenyl)-1,1′-biphenyl (DPVBi) are useful hosts forblue emission.

Useful fluorescent emitting materials include, but are not limited to,derivatives of anthracene, tetracene, xanthene, perylene, rubrene,coumarin, rhodamine, and quinacridone, dicyanomethylenepyran compounds,thiopyran compounds, polymethine compounds, pyrilium and thiapyriliumcompounds, fluorene derivatives, periflanthene derivatives,indenoperylene derivatives, bis(azinyl)amine boron compounds,bis(azinyl)methane compounds, and carbostyryl compounds. Illustrativeexamples of useful materials include, but are not limited to, thefollowing:

L1

L3

L5

L7

L9 X R1 R2 L9 O H H L10 O H Methyl L11 O Methyl H L12 O Methyl MethylL13 O H t-butyl L14 O t-butyl H L15 O t-butyl t-butyl L16 S H H L17 S HMethyl L18 S Methyl H L19 S Methyl Methyl L20 S H t-butyl L21 S t-butylH L22 S t-butyl t-butyl

L2

L4

L6

L8

L10 X R1 R2 L23 O H H L24 O H Methyl L25 O Methyl H L26 O Methyl MethylL27 O H t-butyl L28 O t-butyl H L29 O t-butyl t-butyl L30 S H H L31 S HMethyl L32 S Methyl H L33 S Methyl Methyl L34 S H t-butyl L35 S t-butylH L36 S t-butyl t-butyl

R L37 phenyl L38 methyl L39 t-butyl L40 mesityl

R L41 phenyl L42 methyl L43 t-butyl L44 mesityl

L45

L46

L47

L48

L49

L50

L51

L52Electron-Transporting Layer (ETL)

Preferred thin film-forming materials for use in forming theelectron-transporting layer 111 of the organic EL devices of thisinvention are metal chelated oxinoid compounds, including chelates ofoxine itself (also commonly referred to as 8-quinolinol or8-hydroxyquinoline). Such compounds help to inject and transportelectrons and exhibit both high levels of performance and are readilyfabricated in the form of thin films. Exemplary of contemplated oxinoidcompounds are those satisfying structural formula (E), previouslydescribed.

Other electron-transporting materials include various butadienederivatives as disclosed in U.S. Pat. No. 4,356,429 and variousheterocyclic optical brighteners as described in U.S. Pat. No.4,539,507. Benzazoles satisfying structural formula (G) are also usefulelectron transporting materials. Triazines are also known to be usefulas electron transporting materials.

Other Useful Organic Layers and Device Architecture

In some instances, layers 109 through 111 can optionally be collapsedinto a single layer that serves the function of supporting both lightemission and electron transportation. Layers 110 and 111 may also becollapsed into a single layer that functions to block holes or excitons,and supports electron transportation. It also known in the art thatemitting materials may be included in the hole-transporting layer, whichmay serve as a host. Multiple materials may be added to one or morelayers in order to create a white-emitting OLED, for example, bycombining blue- and yellow-emitting materials, cyan- and red-emittingmaterials, or red-, green-, and blue-emitting materials. White-emittingdevices are described, for example, in EP 1 187 235, U.S. 2002/0025419,EP 1 182 244, U.S. Pat. No. 5,683,823, U.S. Pat. No. 5,503,910, U.S.Pat. No. 5,405,709, and U.S. Pat. No. 5,283,182 and can be equipped witha suitable filter arrangement to produce a color emission.

This invention may be used in so-called stacked device architecture, forexample, as taught in U.S. Pat. No. 5,703,436 and U.S. Pat. No.6,337,492.

Deposition of Organic Layers

The organic materials mentioned above are suitably deposited by anymeans suitable for the form of the organic materials. In the case ofsmall molecules, they are conveniently deposited through sublimation,but can be deposited by other means such as from a solvent with anoptional binder to improve film formation. If the material is a polymer,solvent deposition is usually preferred. The material to be deposited bysublimation can be vaporized from a sublimator “boat” often comprised ofa tantalum material, e.g., as described in U.S. Pat. No. 6,237,529, orcan be first coated onto a donor sheet and then sublimed in closerproximity to the substrate. Layers with a mixture of materials canutilize separate sublimator boats or the materials can be pre-mixed andcoated from a single boat or donor sheet. Patterned deposition can beachieved using shadow masks, integral shadow masks (U.S. Pat. No.5,294,870), spatially-defined thermal dye transfer from a donor sheet(U.S. Pat. No. 5,688,551, U.S. Pat. No. 5,851,709 and U.S. Pat. No.6,066,357) and inkjet method (U.S. Pat. No. 6,066,357).

Encapsulation

Most OLED devices are sensitive to moisture or oxygen, or both, so theyare commonly sealed in an inert atmosphere such as nitrogen or argon,along with a desiccant such as alumina, bauxite, calcium sulfate, clays,silica gel, zeolites, alkaline metal oxides, alkaline earth metaloxides, sulfates, or metal halides and perchlorates. Methods forencapsulation and desiccation include, but are not limited to, thosedescribed in U.S. Pat. No. 6,226,890. In addition, barrier layers suchas SiOx, Teflon, and alternating inorganic/polymeric layers are known inthe art for encapsulation.

Optical Optimization

OLED devices of this invention can employ various well-known opticaleffects in order to enhance its properties if desired. This includesoptimizing layer thicknesses to yield maximum light transmission,providing dielectric mirror structures, replacing reflective electrodeswith light-absorbing electrodes, providing anti-glare or anti-reflectioncoatings over the display, providing a polarizing medium over thedisplay, or providing colored, neutral density, or color-conversionfilters over the display. Filters, polarizers, and anti-glare oranti-reflection coatings may be specifically provided over the cover oras part of the cover.

Embodiments of the invention can provide advantageous features such ashigher luminous yield, lower drive voltage, and higher power efficiency.Embodiments of the organometallic compounds useful in the invention canprovide a wide range of hues including those useful in the emission ofwhite light (directly or through filters to provide multicolordisplays).

The invention and its advantages can be better appreciated by thefollowing examples.

TRIPLET ENERGY CALCULATION EXAMPLE 1

The triplet energy of compound Inv-1 was calculated using theB3LYP/MIDI! method as implemented in the Gaussian98 (Gaussian, Inc.,Pittsburgh, Pa.) computer program. The energies of the ground state,E(gs), and the lowest triplet state, E(ts), were calculated. The energyof each state was computed at the minimum-energy geometry for thatstate. The difference in energy between the two states was furthermodified by the following equation to give the triplet energy:E(t)=0.84*(E(ts)−E(gs))+0.35. This procedure was repeated for theremaining compounds listed in Table 1. The triplet energies in eV andthe wavelength which corresponds to the 0-0 transition (λ₀₋₀) are givenin Table 1. TABLE 1 Calculated Triplet Energies Calculated CalculatedCompound E(t) (eV) λ_(0—0) (nm) Inv-1 2.09 593 Inv-6 2.23 556 Inv-112.55 486 Inv-16 2.81 441

As can be seen from the Table, complexes of the invention offer a widerange of hues and can be used to span the visible spectrum.

SYNTHETIC EXAMPLE 1 Preparation of Ligands

2-Phenylimidazo[1,2-a]pyridine was prepared by the following procedure.A solution of 2-aminopyridine (9.4 g, 0.1 mol) and bromoacetophenone(19.9 g, 0.1 mol) in ethanol (75 mL) was heated to reflux for 2 h. Theethanol was removed in vacuo and the residue redissolved in 100 mL ethylacetate. The solution was washed with 1N NaHCO₃ (50 mL), followed by awater wash. The organics were dried over magnesium sulfate, filtered andthe solvent removed in vacuo. Chromatography on silica withdichloromethane/ethyl acetate (90/10) eluent yielded the2-phenylimidazo[1,2-a]pyridine in 88% yield, mp 134° C. ¹H NMR and massspectra analysis confirmed the structure.

2-Phenylpyrazolo[1,5-a]pyridine was prepared by the following procedure.A solution of 2-(phenylethynyl)pyridine (3.2 g, 0.018 mol) indichloromethane (20 mL) was cooled to 0° C. in an ice bath. A solutionof O-mesitylenesulfonylhydroxylamine in dichloromethane was addeddropwise, keeping the temperature below 5° C. The mixture was stirredfor 30 minutes, then ethyl ether was added to precipitate a solid (5-7g). The solid was filtered off, washed with ethyl ether and dried. Thesolid was re-dissolved in DMF and solid K₂CO₃ (4.5 g, 0.033 mol) wasadded in small portions. After stirring 2 h, water (100 mL) was added tothe mixture, followed by extraction with dichloromethane. The organicswere dried over magnesium sulfate, filtered, and evaporated to an orangeoil (2.3 g). Chromatography on silica with dichloromethane eluentyielded the 2-(phenylethynyl)pyridine in 83% yield, mp 110° C. ¹H NMRand mass spectra analysis confirmed the structure.

SYNTHETIC EXAMPLE 2 Preparation ofBis(2-phenylimidazo[1,2-a]pyridinato-N,C)iridium(III)(acetylacetate)(Inv-24)

K₃IrBr₆ (2.75 g) and 2-phenylimidazo[1,2-a]pyridine (1.69 g) were placedin a 125 mL round-bottom flask. 2-Ethoxy ethanol (45 mL) and deionizedwater (15 mL) were added. The mixture was freeze-thaw degassed andrefluxed 4 hrs under nitrogen atmosphere. After cooling, the resultingprecipitate was filtered in air and washed with ethanol and water togive 1.975 g of yellow-brown powder. The mass spectrum was consistentwith the formulationterakis(2-phenylimidazo[1,2-a]pyridinato)(di-μ-bromo)diiridium(III).This material was used in the next step without further purification.

Tetrakis(2-phenylimidazo[1,2-a]pyridinato)(di-μ-bromo)diiridium(III)(0.654 g) and sodium acetylacetonate hydrate (0.417 g) were placed in a100 mL round bottom flask. 1,2-Dichloroethane (30 mL) was added. Themixture was freeze-thaw degassed and then refluxed under nitrogen 20 h.After cooling, the reaction solution was filtered in air. The productwas precipitated by concentrating and adding a few drops of hexane. Theyellow-brown powder was filtered and dried (0.634 g) to affordbis(2-phenylimidazo[1,2-a]pyridinato-N,C)iridium(III)(acetylacetonate)(Inv-24), which emitted green light in solution. The product wascharacterized by ¹H NMR spectroscopy.

DEVICE EXAMPLE 1

An EL device (Device 1) satisfying the requirements of the invention wasconstructed in the following manner:

A glass substrate coated with an 85-nm layer of indium-tin oxide (ITO)as the anode was sequentially ultrasonicated in a commercial detergent,rinsed in deionized water, degreased in toluene vapor and exposed toultraviolet and ozone for several minutes.

-   -   1. A glass substrate coated with an 85-nm layer of indium-tin        oxide (ITO) as the anode was sequentially ultrasonicated in a        commercial detergent, rinsed in deionized water, degreased in        toluene vapor and exposed to ultraviolet and ozone for several        minutes.    -   2. Over the ITO was deposited a 60-nm PEDOT hole-injecting layer        (HIL) by spin coating an aqueous solution of PEDOT/PSS (1.3% in        water, Baytron® P fromH. C. Stark, Inc.) under controlled        spinning rate of 1750 rpm to obtain the desired layer thickness        of about 60 nm and then annealing the coating on a heated metal        block in air at 120° C. for about 10 min.    -   3. A light-emitting layer (LEL) of poly(9-vinylcarbazole) host        material (PVK, Aldrich Chemical Co., used as received, typical        M_(w) 63,000, typical M_(n) 19,000) was then fabricated.        Compound Inv-24 was combined with the host, PVK, in an amount of        4% by weight relative to the PVK. The mixture was dissolved in        chlorobenzene (2% solids), filtered, and then deposited onto the        hole-injecting layer of PEDOT by spin casting under a controlled        spinning rate of 1500 rpm to obtain the desired layer thickness        of about 66 nm and then baking the coating in an oven at 120° C.        for 20 min.    -   4. On top of LEL was deposited a 1.5-nm layer formed of cesium        fluoride (CsF) by evaporative vacuum deposition.    -   5. On top of the CsF layer was deposited a 200-nm cathode layer        formed of a 10:1 atomic ratio of Mg and Ag by evaporative vacuum        deposition.

The above sequence completed the deposition of the EL device. The devicewas then hermetically packaged in a dry glove box for protection againstambient moisture and oxygen.

DEVICE EXAMPLE 2 (COMPARATIVE)

A second device (Device 2) was fabricated in an identical manner toDevice 1, except that the light-emitting compound Inv-24 was omittedfrom the chlorobenzene solution of the LEL host compound.

The device cells thus prepared were tested for luminance and CIE(Commission Internationale de L'Eclairage) color coordinates at anoperating current of 100 mA/cm². The comparison Device 2 containing nophosphorescent organometallic compound showed the emission from PVKhaving a peak emission at 416 nm and CIE(X,Y) of (0.21,0.14), while theDevice 1 comprising the Inv-24 compound showed the emission at peakwavelength of 520 nm and CIE (X,Y) of (0.37, 0.58). The inventivecompound gave a desirable green hue.

The entire contents of the patents and other publications referred to inthis specification are incorporated herein by reference. The inventionhas been described in detail with particular reference to certainpreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

Parts List

-   101 Substrate-   103 Anode-   105 Hole-Injecting layer (HIL)-   107 Hole-Transporting layer (HTL)-   109 Light-Emitting layer (LEL)-   110 Hole-blocking layer (HBL)-   111 Electron-Transporting layer (ETL)-   113 Cathode

1. An electroluminescent device comprising a light-emitting layerincluding a light emitting material that contains an organometalliccomplex comprising (1) a metal selected from the group consisting of Ir,Rh, Os, Pt, and Pd and (2) a diazole group ligand wherein the ligand hasa fused aromatic ring group including a nitrogen of the diazole as abridgehead nitrogen.
 2. The device of claim 1 wherein the metal is Ir.3. The device of claim 1 wherein the diazole compound is furthersubstituted with a substituent that has at least one double bond.
 4. Thedevice of claim 1 wherein the diazole compound is further substitutedwith a five or six-membered aromatic ring.
 5. The device of claim 1wherein the light emitting material is represented by Formula (1):

wherein: Z represents the atoms necessary to form a diazole ring groupthat is fused with at least one aromatic ring group; N^(f) represents anitrogen atom at a bridgehead position between the diazole ring groupand the fused aromatic ring group; M is a coordinated metal selectedfrom the group consisting of Ir, Rh, Os, Pt, and Pd; m is 1, 2 or 3 whenM is Ir or Rh and m is 1 or 2 when M is Pt, Pd, or Os; L represents anindependently selected ligand group; w is 0-4 as necessary in order tosatisfy a 6 coordination sites when M is Ir, Rh, or Os, and w is 0-2 asnecessary in order to satisfy 4 coordination sites when M is Pt or Pd;and R¹ and R² represent substituent groups, provided that R¹ and R² mayform a ring group.
 6. The device of claim 5 wherein M is Ir.
 7. Thedevice of claim 6 wherein w is 0 and m is
 3. 8. The device of claim 6wherein R¹ and R² represent the atoms necessary to join to form asix-membered aromatic ring group.
 9. The device of claim 5 wherein thelight-emitting material is represented by Formula (1a):

wherein: M, L, w, m, R¹, and R² are as defined in claim 5; R³ representshydrogen or a substituent; and Ar represents the atoms necessary to forman aromatic ring group.
 10. The device of claim 5 wherein thelight-emitting material is represented by Formula (1b):

wherein: M, L, w, m, R¹, and R² are as defined in claim 5; R³ representshydrogen or a substituent; and Ar represents the atoms necessary to forman aromatic ring group.
 11. The device of claim 5 wherein thelight-emitting layer contains a light emitting compound of Formula (1c):

wherein: M, L, w, m, R¹, R² are as defined in claim 5; R³ representshydrogen or a substituent; and Ar represents the atoms necessary to forman aromatic ring group.
 12. The device of claim 5 wherein thelight-emitting layer contains a light emitting compound of Formula (1d):

wherein: M, L, w, m, R¹, R² are as defined in claim 5; R³ representshydrogen or a substituent; and Ar represents the atoms necessary to forman aromatic ring group.
 13. The device of claim 5 wherein thelight-emitting material is represented by Formula (1e):

wherein: Ar represents the atoms necessary to form an aromatic ringgroup; R³ represents hydrogen or a substituent; and Ar² represents theatoms necessary to form a five or six membered aromatic ring group. 14.The device of claim 13 wherein Ar and Ar² independently represent theatoms necessary to form a benzene ring group.
 15. The device of claim 5wherein the light-emitting material is represented by Formula (1f)

wherein: Ar represents the atoms necessary to form an aromatic ringgroup; R³ represents hydrogen or a substituent; and Ar represents theatoms necessary to form a five or six membered aromatic ring group. 16.The device of claim 15 wherein Ar and Ark independently represent theatoms necessary to form a benzene ring group.
 17. The device of claim 5wherein the light-emitting material is represented by Formula (1g):

wherein: Ar represents the atoms necessary to form an aromatic ringgroup; R³ represents hydrogen or a substituent; and Ar² represents theatoms necessary to form a five or six membered aromatic ring group. 18.The device of claim 17 wherein Ar and Ar² independently represent theatoms necessary to form a benzene ring group.
 19. The device of claim 5wherein the light-emitting material is represented by Formula (1h):

wherein: Ar represents the atoms necessary to form an aromatic ringgroup; R³ represents hydrogen or a substituent; and Ar² represents theatoms necessary to form a five or six membered aromatic ring group. 20.The device of claim 19 wherein Ar and Ar² independently represent theatoms necessary to form a benzene ring group.
 21. The device of claim 5wherein the light-emitting layer contains a light emitting material ofFormula (1a), (1b), (1c), or (1d).

wherein: M is a coordinated metal selected from the group consisting ofIr, Rh, Pt, Os, and Pd; m is 1, 2, or 3 when M is Ir or Rh and m is 1 or2 when M is Pt, Pd, or Os; L represents an independently selected ligandgroup; w is 0-4 as necessary in order to satisfy a 6 coordination siteswhen M is Ir, Os, or Rh and w is 0-2 as necessary in order to satisfy 4coordination sites when M is Pt or Pd; and R¹ and R² representsubstituent groups, provided that R¹ and R² may join to form a ringgroup; and R³ represents hydrogen or a substituent; Ar represents theatoms necessary to form an aromatic ring group.
 22. An organometalliccomplex comprised of: Ir, Rh, Os, Pt, or Pd and a diazole group ligandwherein the ligand has a fused aromatic ring group including a nitrogenof the diazole as a bridgehead nitrogen.
 23. An organometallic complexaccording to claim 22 represented by Formula (1),

wherein: Z represents the atoms necessary to form a diazole group ligandwherein the ligand has a fused aromatic ring group including a nitrogenof the diazole as a bridgehead nitrogen; N^(f) represents a nitrogenatom at a bridgehead position between the diazole ring group and thefused aromatic ring group; M is a coordinated metal selected from thegroup consisting of Ir, Rh, Pt, Os, and Pd; m is 1, 2, or 3 when M is Iror Rh and m is 1 or 2 when M is Pt, Pd, or Os; L represents anindependently selected ligand group; the sum of w and m is 3 when M iskr, Rh, or Os and the sum of w and m is 2 when M is Pt or Pd; and R¹ andR² represent substituent groups, provided that R¹ and R² may join toform a ring group and R¹ and R² may be chosen so as to fuse with thediazole ring group.
 24. An organometallic complex according to claim 22represented by Formula (1a), (1b), (1c), or (1d):

wherein: M is a coordinated metal selected from the group consisting ofIr, Rh, Pt, Pd. and Os; m is 1, 2, or 3 when M is Ir or Rh and m is 1 or2 when M is Pt, Pd, or Os; L represents an independently selected ligandgroup; w is such that the sum of w and m is 3 when M is Ir, Rh, or Osand the sum of w and m is 2 when M is Pt or Pd; and R¹ and R² representsubstituent groups, provided that R¹ and R² may join to form a ringgroup; R³ represents hydrogen or a substituent; and Ar represents theatoms necessary to form an aromatic ring group.
 25. The device of claim1 wherein the emitting material is a dopant compound disposed in a hostmaterial.
 26. The device of claim 25 wherein the dopant compound ispresent in an amount of up to 15 wt % based on the host.
 27. The deviceof claim 1 wherein the light-emitting material is part of a polymer. 28.The device of claim 1 including a means for emitting white light. 29.The device of claim 28 including a filtering means.
 30. The device ofclaim 1 including a fluorescent emitting material.
 31. A displaycomprising the OLED device of claim
 1. 32. An area lighting devicecomprising the OLED device of claim
 1. 33. A process for emitting lightcomprising applying a potential across the device of claim 1.